The most common and most likely answer to this crossword clue is the 15 letter word DIFFERENTIATION
I've seen this crossword clue in Financial Times
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More clues leading to the result DIFFERENTIATION
Buddy explains!
The straight part
We know that "Exact definition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case exact definition = differentiation.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DIFFERENTIATION mean?
DIFFERENTIATION noun- The act of differentiating.
- The act of distinguishing or describing a thing, by giving its different, or specific difference; exact definition or determination.
- The gradual formation or production of organs or parts by a process of evolution or development, as when the seed develops the root and the stem, the initial stem develops the leaf, branches, and flower buds; or in animal life, when the germ evolves the digestive and other organs and members, or when the animals as they advance in organization acquire special organs for specific purposes.
- (geology) The process of separation of cooling magma into various rock types.
- The process of determining the derived function of a function.